Lithium-cobalt based complex oxide having superior lifespan characteristics and cathode active material for secondary batteries including the same

ABSTRACT

Disclosed is a lithium-cobalt based complex oxide represented by Formula 1 below including lithium, cobalt and manganese wherein the lithium-cobalt based complex oxide maintains a crystal structure of a single O3 phase at a state of charge (SOC) of 50% or more based on a theoretical amount:
 
Li x Co 1-y-z Mn y A z O 2   (1)
         wherein 0.95≤x≤1.15, 0≤y≤0.3 and 0≤z≤0.2; and   A is at least one element selected the group consisting of Al, Mg, Ti, Zr, Sr, W, Nb, Mo, Ga, and Ni.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a national phase entry under 35 U.S.C. § 371 ofInternational Application No. PCT/KR2014/007657, filed Aug. 19, 2014,which claims priority to Korean Patent Application No. 10-2013-0097828,filed Aug. 19, 2013, the disclosures of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a lithium-cobalt based complex oxidehaving superior lifespan characteristics and a cathode active materialfor secondary batteries including the same. More particularly, thepresent invention relates to a lithium-cobalt based complex oxideincluding lithium and cobalt maintaining a crystal structure of a singleO3 phase at a state of charge (SOC) of 50% or more.

BACKGROUND ART

As mobile device technology continues to develop and demand thereforcontinues to increase, demand for secondary batteries as energy sourcesis rapidly increasing. Among these secondary batteries, lithiumsecondary batteries, which have high energy density and operatingvoltage, long cycle lifespan, and low self-discharge rate, arecommercially available and widely used.

In addition, recently, lithium ion batteries are commercially used as apower supply in home electronics such as laptop computers, mobile phonesand the like. Furthermore, as interest in environmental problems isincreasing, research into electric vehicles (EVs), hybrid electricvehicles (HEVs), and the like that can replace vehicles using fossilfuels, such as gasoline vehicles, diesel vehicles, and the like, whichare one of the main causes of air pollution, is underway.

As a cathode of conventionally used lithium ion batteries, lithiumcobalt oxides such as LiCoO₂ having a layered structure are used. As ananode, graphite based materials are generally used.

Lithium cobalt oxides are currently widely used due to superior physicalproperties such as superior cycle characteristics as compared to LiNiO₂and LiMn₂O₄. To develop secondary batteries having high energy density,cathode active materials having large capacity are required. However,when operating voltage of lithium cobalt oxides are fixed unlike threecomponent-based cathode active materials, it is substantially impossibleto enlarge capacities of materials.

Accordingly, lithium cobalt oxides must be used under high voltage todevelop secondary batteries having high energy density. However,approximately 50% or more of lithium ions are eliminated under highvoltage operation, structures of lithium cobalt oxides collapse and, assuch, lifespan characteristics are rapidly degraded.

To overcome this problem and to achieve high energy density,technologies substituting some cobalt with Al, Mg, B or the like, ortreating surfaces of lithium cobalt oxides with a metal oxide such asAl₂O₃, Mg₂O, TiO₂ or the like are known.

However, when some cobalt is substituted with metals described above,there is still a problem such as degradation of lifespancharacteristics. When a surface of a lithium cobalt oxide is coated witha metal oxide, specific capacity may be reduced due to addition of acoating material that does not directly participate in charge anddischarge reaction, and a metal oxide with very low electricalconductivity mainly constitutes the coating material, which results inreduced conductivity. In addition, the coating process reduces activereaction area, thereby increasing interfacial resistance anddeteriorating high-rate charge and discharge characteristics.

Therefore, there is an urgent need to develop technology forfundamentally addressing these problems and enhancing high voltagelifespan characteristics of a lithium cobalt oxide.

DISCLOSURE Technical Problem

The present invention aims to address the aforementioned problems of therelated art and to achieve technical goals that have long been sought.

As a result of a variety of intensive studies and various experiments,the inventors of the present invention confirmed that ratecharacteristics and lifespan characteristics are improved when alithium-cobalt based complex oxide maintains a crystal structure of asingle O3 phase at a state of charge (SOC) of 50% or more, namely, underhigh voltage, thus completing the present invention.

Technical Solution

In accordance with one aspect of the present invention, provided is alithium-cobalt based complex oxide including lithium, cobalt andmanganese, represented by Formula 1 below, wherein the lithium-cobaltbased complex oxide maintains a crystal structure of a single O3 phaseat a state of charge (SOC) of 50% or more based on a theoretical amount:Li_(x)Co_(1-y-z)Mn_(y)A_(z)O₂  (1)

wherein 0.95≤x≤1.15, 0≤y≤0.3 and 0≤z≤0.2; and

A is at least one element selected the group consisting of Al, Mg, Ti,Zr, Sr, W, Nb, Mo, Ga, and Ni.

The lithium-cobalt based complex oxide may maintain a crystal structureof a single O3 phase even when delithiation progresses in 50% or more ofthe lithium-cobalt based complex oxide.

As referred to above, when delithiation progresses in 50% or more ofgeneral lithium-cobalt based complex oxides, structures of thelithium-cobalt based complex oxides collapse and thereby it is difficultto maintain charging and discharging. Accordingly, lifespancharacteristics are degraded.

When the lithium-cobalt based complex oxide has the O3 phase crystalstructure, charging and discharging may be stably repeated. However,when the lithium-cobalt based complex oxide has a P3 phase crystalstructure, lifespan may be deteriorated.

On the other hand, although delithiation progresses in 50% or more ofthe lithium-cobalt based complex oxide according to the presentinvention, the lithium-cobalt based complex oxide may maintain a crystalstructure of a single O3 phase. Accordingly, even when lithium ions areintensively eliminated under high voltage, the lithium cobalt complexoxide may be charged and discharged without structural collapse.Accordingly, lifespan characteristics are improved.

In secondary batteries, generally, high voltage is considered anoperating voltage exceeding 4.3 V. In one specific embodiment, thelithium-cobalt based complex oxide according to the present inventionmay maintain the crystal structure of the single O3 phase underoperating voltage of 4.35 V or more, more particularly operating voltageof 4.35 V or more to 4.5 V or less.

Meanwhile, the lithium-cobalt based complex oxide may have a crystalstructure including two phases, namely, O3 phase and P3 phase underoperating voltage exceeding 4.5 V.

Inventors of the present invention confirmed that, when, as describedabove, some cobalt is doped with Mn, a lithium-cobalt based complexoxide may maintain a crystal structure of a single O3 phase although alarge amount of lithium ions are eliminated in the lithium-cobalt basedcomplex oxide, compared a lithium-cobalt based complex oxide not dopedwith Mn. Accordingly, under high operating voltage, a structure is notcollapsed and, as such, lifespan characteristics are improved.

When a doping amount of Mn is greater than 30 mol %, the capacity of asecondary battery is reduced and the morphology of the lithium-cobaltbased complex oxide is changed. In addition, a surface is not smoothand, as such, tapped density is lowered or specific surface area isenlarged. In addition, problems may occur during processing. A dopingamount of Mn, y, may be particularly 0.0001≤y≤0.2, more particularly0.001≤y≤0.1.

When a doping amount of Mn is greater than 0.3 mol %, the capacity of asecondary battery is reduced and the morphology of the lithium-cobaltbased complex oxide is changed. In addition, a surface is not smoothand, as such, tapped density is lowered or specific surface area isenlarged. In addition, problems may occur during processing. A dopingamount of Mn, y, may be particularly 0.0001≤y≤0.2, more particularly0.001≤y≤0.1.

In one specific embodiment, the amount of the added element A, the z,may be particularly 0≤z≤0.2, A may be at least one selected from thegroup consisting of Al, Ti, Ni and Mg, particularly Mg.

As described above, the inventors of the present application confirmedthat, when the lithium-cobalt based complex oxide is doped with Mn andthe additional element A, lifespan characteristics are improved.

In one specific embodiment, an average particle size of thelithium-cobalt based complex oxide according to the present inventionmay be 0.5 micrometers to 30 micrometers, particularly 3 micrometers to25 micrometers.

When the average particle diameter is within the above range, densitymay be improved and, by a proper particle diameter combination, ratecharacteristics and electrochemical characteristics may be maintained.Whereas, when average particle diameter is greater than the aboveparticle diameters, rate characteristics and capacity may be lowered.When average particle diameter is less than the above particlediameters, problems may occur during a cathode slurry manufacturingprocess.

The present invention also provides a cathode active material includingthe lithium-cobalt based complex oxide, a cathode mixture for secondarybatteries including the cathode mixture, and a cathode for secondarybatteries including the cathode mixture.

The cathode active material may include, in addition to thelithium-cobalt based complex oxide, a layered structure compound such aslithium nickel oxide (LiNiO₂) or the like, or a compound substitutedwith one or more transition metals; a lithium manganese oxide such asformula Li_(1+x)Mn_(2−x)O₄ where x is 0˜0.33, LiMnO₃, LiMn₂O₃, LiMnO₂ orthe like; lithium copper oxide (Li₂CuO₂); vanadium oxides such asLiV₃O₈, LiV₃O₄, V₂O₅, Cu₂V₂O₇ or the like; a Ni site type lithium nickeloxide represented by formula LiNi_(1−x)M_(x)O₂ where M is Co, Mn, Al,Cu, Fe, Mg, B or Ga, and x is 0.01 to 0.3); a lithium manganese complexoxide represented by formula LiMn_(2−x)M_(x)O₂ where M is Co, Ni, Fe,Cr, Zn or Ta, and x is 0.01 to 1 or Li₂Mn₃MO₈ where M is Fe, Co, Ni, Cuor Zn; a lithium manganese complex oxide of a spinel structurerepresented by LiNi_(x)Mn_(2−x)O₄; LiMn₂O₄ where some Li is substitutedwith alkaline earth metals; disulfide compounds; Fe₂(MoO₄)₃; and thelike.

Here, in addition to the lithium-cobalt based complex oxide, materialsmay be included in a range of, for example, 0.1 to 80%, particularly 1to 60%, more particularly 1 to 50%, based on the total weight of thecathode active material.

The cathode mixture may further selectively include a conductivematerial, a binder, a filler, and the like, in addition to the cathodeactive material.

The conductive material is typically added in an amount of 1 to 30 wt %based on the total weight of the mixture including the cathode activematerial. There is no particular limit as to the conductive material, solong as it does not cause chemical changes in the fabricated battery andhas conductivity. Examples of conductive materials include graphite suchas natural or artificial graphite; carbon black such as carbon black,acetylene black, Ketjen black, channel black, furnace black, lamp black,and thermal black; conductive fibers such as carbon fibers and metallicfibers; metallic powders such as carbon fluoride powder, aluminumpowder, and nickel powder; conductive whiskers such as zinc oxide andpotassium titanate; conductive metal oxides such as titanium oxide; andpolyphenylene derivatives.

The binder is a component assisting in binding between an activematerial and the conductive material and in binding of the activematerial to a current collector. The binder is typically added in anamount of 1 to 30 wt % based on the total weight of the mixtureincluding the cathode active material. Examples of the binder include,but are not limited to, polyvinylidene fluoride, polyvinyl alcohols,carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene,polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM),sulfonated EPDM, styrene butadiene rubber, fluorine rubber, and variouscopolymers.

The filler is optionally used as a component to inhibit cathodeexpansion. The filler is not particularly limited so long as it is afibrous material that does not cause chemical changes in the fabricatedbattery. Examples of the filler include olefin-based polymers such aspolyethylene and polypropylene; and fibrous materials such as glassfiber and carbon fiber.

The cathode according to the present invention may be manufactured bycoating, on a cathode current collector, a slurry prepared by mixing thecathode mixture including the above-described compounds with a solventsuch as NMP or the like and drying and pressing the coated cathodecurrent collector.

The cathode current collector is generally fabricated to a thickness of3 to 500 μm. The cathode current collector is not particularly limitedso long as it does not cause chemical changes in the fabricated lithiumsecondary battery and has conductivity. For example, the cathode currentcollector may be made of stainless steel, aluminum, nickel, titanium,sintered carbon, aluminum or stainless steel surface-treated withcarbon, nickel, titanium, or silver, or the like. The cathode currentcollector may have fine irregularities at a surface thereof to increaseadhesion between the cathode active material and the cathode currentcollector. In addition, the cathode current collector may be used in anyof various forms including films, sheets, foils, nets, porousstructures, foams, and non-woven fabrics.

The present invention also provides a lithium secondary batteryincluding the cathode, an anode, a separator, and a lithiumsalt-containing non-aqueous electrolyte.

The anode may be manufactured by, for example, coating an anode mixtureincluding an anode active material on an anode current collector anddrying the coated anode current collector. As desired, the anode mixturemay further include the above-described components.

Examples of the anode active material include carbon such as hard carbonand graphite-based carbon; metal composite oxides such as Li_(x)Fe₂O₃where 0≤x≤1, Li_(x)WO₂ where 0≤x≤1, Sn_(x)Me_(1−x)Me′_(y)O_(z) where Me:Mn, Fe, Pb, or Ge; Me′: Al, B, P, Si, Groups I, II and III elements, orhalogens; 0<x≤1; 1≤y≤3; and 1≤z≤8; lithium metals; lithium alloys;silicon-based alloys; tin-based alloys; metal oxides such as SnO, SnO₂,PbO, PbO₂, Pb₂O₃, Pb₃O₄, Sb₂O₃, Sb₂O₄, Sb₂O₅, GeO, GeO₂, Bi₂O₃, Bi₂O₄,and Bi₂O₅; conductive polymers such as polyacetylene; and Li—Co—Ni-basedmaterials.

The anode current collector is typically fabricated to a thickness of 3to 500 μm. The anode current collector is not particularly limited solong as it does not cause chemical changes in the fabricated battery andhas high conductivity. For example, the anode current collector may bemade of copper, stainless steel, aluminum, nickel, titanium, sinteredcarbon, copper or stainless steel surface-treated with carbon, nickel,titanium, or silver, and aluminum-cadmium alloys. Similar to the cathodecurrent collector, the anode current collector may also have fineirregularities at a surface thereof to enhance adhesion between theanode current collector and the anode active material and be used invarious forms including films, sheets, foils, nets, porous structures,foams, and non-woven fabrics.

The separator is disposed between the cathode and the anode and, as theseparator, a thin insulating film with high ion permeability and highmechanical strength is used. The separator generally has a pore diameterof 0.01 to 10 μm and a thickness of 5 to 300 μm. As the separator, forexample, sheets or non-woven fabrics, made of an olefin-based polymersuch as polypropylene; or glass fibers or polyethylene, which havechemical resistance and hydrophobicity, are used. When a solidelectrolyte such as a polymer or the like is used as an electrolyte, thesolid electrolyte may also serve as a separator.

The lithium salt-containing non-aqueous electrolyte consists of anelectrolyte and a lithium salt. The electrolyte may be a non-aqueousorganic solvent, an organic solid electrolyte, an inorganic solidelectrolyte, or the like.

Examples of the non-aqueous organic solvent include non-protic organicsolvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylenecarbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate,gamma-butyrolactone, 1,2-dimethoxy ethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide,dimethylformamide, dioxolane, acetonitrile, nitromethane, methylformate, methyl acetate, phosphoric acid triester, trimethoxy methane,dioxolane derivatives, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, methyl propionate, and ethylpropionate.

Examples of the organic solid electrolyte include polyethylenederivatives, polyethylene oxide derivatives, polypropylene oxidederivatives, phosphoric acid ester polymers, polyagitation lysine,polyester sulfide, polyvinyl alcohols, polyvinylidene fluoride, andpolymers containing ionic dissociation groups.

Examples of the inorganic solid electrolyte include, but are not limitedto, nitrides, halides and sulfates of lithium (Li) such as Li₃N, LiI,Li₅NI₂, Li₃N—LiI—LiOH, LiSiO₄, LiSiO₄—LiI—LiOH, Li₂SiS₃, Li₄SiO₄,Li₄SiO₄—LiI—LiOH, and Li₃PO₄—Li₂S—SiS₂.

The lithium salt is a material that is readily soluble in thenon-aqueous electrolyte and examples thereof include, but are notlimited to, LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃,LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi,chloroborane lithium, lower aliphatic carboxylic acid lithium, lithiumtetraphenyl borate, and imides.

In addition, in order to improve charge/discharge characteristics andflame retardancy, for example, pyridine, triethylphosphite,triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphorictriamide, nitrobenzene derivatives, sulfur, quinone imine dyes,N-substituted oxazolidinone, N,N-substituted imidazolidine, ethyleneglycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol,aluminum trichloride or the like may be added to the electrolyte. Ifnecessary, in order to impart incombustibility, the electrolyte mayfurther include halogen-containing solvents such as carbon tetrachlorideand ethylene trifluoride. Further, in order to improve high-temperaturestorage characteristics, the non-aqueous electrolyte may further includecarbon dioxide gas, fluoro-ethylene carbonate (FEC), propene sultone(PRS), fluoro-propylene carbonate (FPC), or the like.

The secondary battery according to the present invention may be used ina battery cell used as a power source of small devices such as wirelessdevices, mobile phones, tablet PCs, laptop computers, radios or the likeand may also be used as a unit battery of a battery module including aplurality of battery cells.

The present invention also provides a device including the secondarybattery or battery module. Examples of the device include, but are notlimited to, electric vehicles (EVs), hybrid EVs (HEVs), and plug-in HEVs(PHEVs); and devices for storing power.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanying drawing,in which:

FIG. 1 is a graph comparing capacity retention ratios at 25□ accordingto Experimental Example 2 of the present invention;

FIG. 2 is a graph comparing capacity retention ratios at 45□ accordingto Experimental Example 2 of the present invention;

FIG. 3 is a graph illustrating lifespan characteristics of Example 1,per cycle, at 25□ according to Experimental Example 2 of the presentinvention;

FIG. 4 is a graph illustrating lifespan characteristics of Example 2,per cycle, at 25□ according to Experimental Example 2 of the presentinvention;

FIG. 5 is a graph illustrating lifespan characteristics of Example 3,per cycle, at 25□ according to Experimental Example 2 of the presentinvention;

FIG. 6 is a graph illustrating lifespan characteristics of ComparativeExample 1, per cycle, at 25□ according to Experimental Example 2 of thepresent invention;

FIG. 7 is a graph illustrating lifespan characteristics of Example 1,per cycle, at 45□ according to Experimental Example 2 of the presentinvention;

FIG. 8 is a graph illustrating lifespan characteristics of Example 2,per cycle, at 45□ according to Experimental Example 2 of the presentinvention;

FIG. 9 is a graph illustrating lifespan characteristics of Example 3,per cycle, at 45□ according to Experimental Example 2 of the presentinvention;

FIG. 10 is a graph illustrating lifespan characteristics of ComparativeExample 1, per cycle, at 25□ according to Experimental Example 2 of thepresent invention;

FIG. 11 is a graph comparing rate characteristics according toExperimental Example 2 of the present invention; and

FIG. 12 is a graph comparing capacity retention ratios at 45□ accordingto Experimental Example 3.

BEST MODE

Now, the present invention will be described in more detail withreference to the following examples. These examples are provided onlyfor illustration of the present invention and should not be construed aslimiting the scope and spirit of the present invention.

Example 1

A Co source, Mn source and Li source were sufficiently mixed using a drymethod to obtain a mixture including Li:Co:Mn:O in a molar ratio of1.03:0.9982:0.0018:2. The mixture was plasticized at 900˜1200□ for 5 to20 hours. The obtained product was ground and classified. As a result, alithium-cobalt based complex oxide represented byLi_(1.03)Co_(0.9989)Mn_(0.0011)O₂ was obtained.

Example 2

A Co source, Mn source and Li source were sufficiently mixed using a drymethod to obtain a mixture including Li:Co:Mn:O in a molar ratio of1.03:0.9969:0.0031:2. The mixture was plasticized at 900˜1200□ for 5 to20 hours. The obtained product was ground and classified. As a result, alithium-cobalt based complex oxide represented byLi_(1.03)Co_(0.9982)Mn_(0.0018)O₂ was obtained.

Example 3

A Co source, Mn source and Li source were sufficiently mixed using a drymethod to obtain a mixture including Li:Co:Mn:O in a molar ratio of1.03:0.9957:0.0043:2. The mixture was plasticized at 900˜1200□ for 5 to20 hours. The obtained product was ground and classified. As a result, alithium-cobalt based complex oxide represented byLi_(1.03)Co_(0.9975)Mn_(0.0025)O₂ was obtained.

Example 4

A Co source, Mn source and Li source were sufficiently mixed using a drymethod to obtain a mixture including Li:Co:Mn:O in a molar ratio of1.03:0.9933:0.0067:2. The mixture was plasticized at 900˜1200□ for 5 to20 hours. The obtained product was ground and classified. As a result, alithium-cobalt based complex oxide represented byLi_(1.03)Co_(0.9960)Mn_(0.0040)O₂ was obtained.

Example 5

A Co source, Mn source and Li source were sufficiently mixed using a drymethod to obtain a mixture including Li:Co:Mn:O in a molar ratio of1.03:0.9519:0.0481:2. The mixture was plasticized at 900˜1200□ for 5 to20 hours. The obtained product was ground and classified. As a result, alithium-cobalt based complex oxide represented byLi_(1.03)Co_(0.9550)Mn_(0.0450)O₂ was obtained.

Example 6

A Co source, Mn source, Mg source and Li source were sufficiently mixedusing a dry method to obtain a mixture including Li:Co:Mn:Mg:O in amolar ratio of 1.03:0.9988:0.0011:0.0001:2. The mixture was plasticizedat 900˜1200□ for 5 to 20 hours. The obtained product was ground andclassified. As a result, a lithium-cobalt based complex oxiderepresented by Li_(1.03)Co_(0.9988)Mn_(0.0011)Mg_(0.0001)O₂ wasobtained.

Example 7

A Co source, Mn source, Mg source and Li source were sufficiently mixedusing a dry method to obtain a mixture including Li:Co:Mn:Mg:O in amolar ratio of 1.03:0.9986:0.0011:0.0003:2. The mixture was plasticizedat 900˜1200□ for 5 to 20 hours. The obtained product was ground andclassified. As a result, a lithium-cobalt based complex oxiderepresented by Li_(1.03)Co_(0.9986)Mn_(0.0011)Mg_(0.0003)O₂ wasobtained.

Comparative Example 1

A Co source and Li source were sufficiently mixed using a dry method toobtain a mixture including Li:Co:O in a molar ratio of 1.03:1:2. Themixture was plasticized at 900˜1200

for 5 to 20 hours. The obtained product was ground and classified. As aresult, a lithium-cobalt based complex oxide represented byLi_(1.03)CoO₂ was obtained.

Experimental Example 1

Lithium-cobalt based complex oxide samples manufactured according toExamples 1 and 4, and Comparative Example 1 were prepared. An X-raydiffraction (XRD) pattern of each sample was collected using a SiemensD500 diffractometer equipped with copper target X-ray tube anddiffracted beam monochromator. Since the samples are thick and wide, thesamples were manufactured in a flat and rectangular powder-bed shapesuch that volume irradiated by X-ray beam is constant. Using GSAS of aRietveld refinement program disclosed in [A. C. Larson and R. B. VonDreele, “General Structure Analysis System (GSAS)”, Los Alamos NationalLaboratory Report LAUR 86-748 (2000)], a lattice constant of a unit cellwas calculated. Results are summarized in Table 1 below.

Here, crystal structures of unit cells manufactured according to Example1 and Comparative Example 1 were measured in a full voltage range. Thelithium-cobalt based complex oxide sample manufactured according toExample 4 maintained a crystal structure of a single O3 phase in voltageof 4.4 V or more. A crystal structure was not measured below the voltagerange.

TABLE 1 Voltage (V) Cell parameter 4.3 4.35 4.4 4.45 4.5 Comparative a2.809 2.809 2.812 2.810 2.811 2.812 2.810 2.812 Example 1 c 14.45 14.4414.40 14.43 14.39 14.40 14.36 14.34 Example 1 a 2.809 2.810 2.810 2.8102.811 c 14.44 14.43 14.42 14.41 14.40 Example 4 a — — 2.810 2.811 2.812c — — 14.42 14.42 14.40

Referring to Table 1, the lithium-cobalt based oxides manufacturedaccording to Examples 1 and 4 maintained crystal structures of single O3phases under full charging voltage not exceeding 4.50 V. On the otherhand, the lithium-cobalt based complex oxide manufactured according toComparative Example 1 showed another phase, in addition to an O3 phase,over 4.35 V, resulting in two phases. Subsequently, at 4.50 V, all O3phases transitioned to the another phase, resulting in formation of onephase.

Experimental Example 2

Using each of the lithium-cobalt based complex oxides manufacturedaccording to Example 1 to 3 and Comparative Example 1, thelithium-cobalt based complex oxide:a conductive material (Denka black):abinder (PVdF) in a weight ratio of 95:2.5:2.5 were added to NMP and thenmixed to manufacture a cathode mixture. The cathode mixture was coatedto a thickness of 200 μm on an aluminum foil and then pressed and dried.As a result, a cathode was manufactured.

To manufacture a lithium secondary battery, Li metal was used as ananode and a carbonate based electrolyte, namely, 1 mol LiPF₆ dissolvedin a mixture of ethyl carbonate (EC) and ethyl methyl carbonate (EMC)mixed in a ratio of 1:1 was used as an electrolyte.

Measurement of Initial Charge and Discharge Capacities, and Efficiencies

When the manufactured lithium secondary batteries were charged anddischarged at 0.1 C in a voltage range of 3.0 V to 4.4 V, initialcapacities and efficiencies were measured. Results are summarized inTable 2 below.

Measurement of Lifespan Characteristics

After charging and discharging the manufactured lithium secondarybatteries once at 0.1 C in chambers of 25□ and in a voltage range of 3.0V to 4.4 V, lifespan characteristics were measured fifty times whilecharging at 0.5 C and discharging at 1 C. After charging and dischargingonce at 0.1 C in a 45□ chamber and in a voltage range of 3.0 V to 4.5 V,lifespan characteristics were measured fifty times while charging at 0.5C and discharging at 1 C. Results are summarized in Table 2 below andillustrated in FIGS. 1 to 10.

Measurement of Rate Characteristics

Rate characteristics of the manufactured lithium secondary batterieswere tested in a voltage range of 3.0 V to 4.4 V and capacity at eachC-rate with respect to capacities at 0.1 C was calculated. Results aresummarized in Table 2 below and illustrated in FIG. 11.

TABLE 2 Lifespan characteristics Initial capacity and efficiency Rate(%, at 50 cycles) Charge Discharge Efficiency characteristics 3.0~4.4 V3.0~4.5 V (mAh/g) (mAh/g) (%) 1.0 C 2.0 C (25° C.) (45° C.) Example 1180.5 177.4 98.2 98.0 95.9 97.4 94.4 Example 2 180.3 177.1 98.3 98.496.8 98.2 97.1 Example 3 179.6 176 98.0 99.2 98.2 98.3 97.2 Comparative180.9 176.8 97.8 93.9 88.8 89.4 67.9 Example 1

Referring to Table 2 and FIGS. 1 to 10, initial capacities andefficiencies of lithium secondary batteries using the lithium-cobaltbased complex oxides manufactured according to Examples 1 to 3 wereslightly higher but were not greatly different, when compared to thoseof a lithium secondary battery using the lithium-cobalt based complexoxide manufactured according to Comparative Example 1. However, ratecharacteristics and lifespan characteristics of the lithium secondarybatteries using the lithium-cobalt based complex oxides manufacturedaccording to Examples 1 to 3 were superior, when compared to those of alithium secondary battery using the lithium-cobalt based complex oxidemanufactured according to Comparative Example 1. In particular, ratecharacteristics at a high rate and lifespan characteristics at hightemperature were vastly superior.

As described in Experimental Example 1, the lithium-cobalt based complexoxide manufactured according to Example 1 maintained the crystalstructure of the single O3 phase even under high voltage. On the otherhand, the O3 phase of the lithium-cobalt based complex oxidemanufactured according to Comparative Example 1 is partially or entirelychanged into the P3 phase and thereby charge and discharge are notmaintained and irreversible capacity increases.

For reference, in FIGS. 1 and 2, graphs of Example 2 overlap with graphsof Example 3 and thereby the graphs are not easily distinguished.

Experimental Example 3

Using each of the lithium-cobalt based complex oxides manufacturedaccording to Examples 1, 6 and 7, the lithium-cobalt based complexoxide:a conductive material (Denka black):a binder (PVdF) in a weightratio of 95:2.5:2.5 were added to NMP and then mixed to manufacture acathode mixture. The cathode mixture was coated to a thickness of 200 μmon an aluminum foil and then pressed and dried. As a result, a cathodewas manufactured.

To manufacture a lithium secondary battery, Li metal was used as ananode and a carbonate based electrolyte, namely, 1 mol LiPF₆ dissolvedin a mixture of ethyl carbonate (EC) and ethyl methyl carbonate (EMC)mixed in a ratio of 1:1 was used as an electrolyte.

After charging and discharging the manufactured lithium secondarybatteries once at 0.1 C in a 45□ chamber and in a voltage range of 3.0 Vto 4.5 V, lifespan characteristics were measured fifty times whilecharging at 0.5 C and discharging at 1 C. Results are illustrated inFIG. 12.

Referring to FIG. 12, the lithium-cobalt based complex oxides, which aredoped with Mg, manufactured according to Examples 6 and 7 showedexcellent lifespan characteristics, when compared with thelithium-cobalt based complex oxide manufactured according to Example 1.

Those skilled in the art will appreciate that various modifications,additions and substitutions are possible, without departing from thescope and spirit of the invention as disclosed in the accompanyingclaims.

INDUSTRIAL APPLICABILITY

As described above, a lithium-cobalt based complex oxide according tothe present invention maintains a crystal structure of a single O3 phaseat a state of charge (SOC) of 50% or more, namely, under high voltageand thereby collapse of a structure of the lithium-cobalt based complexoxide is prevented, and, accordingly, rate characteristics and lifespancharacteristics are improved.

The invention claimed is:
 1. A lithium-cobalt based complex oxiderepresented by Formula 1 below comprising lithium, cobalt and manganese,wherein the lithium-cobalt based complex oxide maintains a crystalstructure of a single O3 phase at a state of charge (SOC) of 50% or morebased on theoretical capacity:Li_(x)Co_(1-y-z)Mn_(y)A_(z)O₂  (1) wherein 0.95≤x≤1.15, 0<y<0.045 and0<z≤0.0003; and A is at least one element selected the group consistingof Al, Mg, Ti, and Ni, wherein at least one element of A is Mg.
 2. Thelithium-cobalt based complex oxide according to claim 1, wherein thelithium-cobalt based complex oxide maintains the crystal structure ofthe single O3 phase when delithiation progresses in 50% or more of thelithium-cobalt based complex oxide.
 3. The lithium-cobalt based complexoxide according to claim 1, wherein the lithium-cobalt based complexoxide maintains the crystal structure of the single O3 phase underoperating voltage of 4.35 V or more.
 4. The lithium-cobalt based complexoxide according to claim 3, wherein the lithium-cobalt based complexoxide maintains the crystal structure of the single O3 phase underoperating voltage of 4.35 V or more to 4.5 V or less.
 5. Thelithium-cobalt based complex oxide according to claim 1, wherein anaverage particle size of the lithium-cobalt based complex oxide is 0.5micrometers to 30 micrometers.
 6. A cathode active material comprisingthe lithium-cobalt based complex oxide according to claim
 1. 7. Acathode mixture for secondary batteries comprising the cathode activematerial according to claim
 6. 8. A cathode for secondary batteriescomprising the cathode mixture for secondary batteries according toclaim 7 coated on a collector.
 9. A lithium secondary battery comprisingthe cathode for secondary batteries according to claim
 8. 10. A batterymodule comprising the lithium secondary battery according to claim 9 asa unit battery.
 11. A device comprising the lithium secondary batteryaccording to claim
 9. 12. A device comprising the battery moduleaccording to claim
 10. 13. The device according to claim 11, wherein thedevice is a mobile phone, a tablet PC, a laptop computer, an electricvehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle,or a system for storing power.
 14. The device according to claim 12,wherein the device is a mobile phone, a tablet PC, a laptop computer, anelectric vehicle, a hybrid electric vehicle, a plug-in hybrid electricvehicle, or a system for storing power.